“It’s not just a coincidence that so much of my family is devoted to public service,” says Rory Kennedy, who, as Robert Kennedy’s youngest daughter, has clearly inherited the social-justice gene, making documentaries on such subjects as rural poverty and AIDS. Her latest film, A Boy’s Life (airing March 24 on HBO), is about a troubled Mississippi 7-year-old who’s caught in a custody dispute between his chronically ill mother and his domineering grandmother. At one point, the grandmother claims the boy tried to kill himself; later, in a very unsettling scene, she hands him a gun to play with. (It wasn’t loaded.) It was a moment that tested Kennedy’s noninterventionist tendencies as a documentarian. “In that scene, I felt the most visceral reaction of I’ve got to do something,” says the filmmaker, who has a 16-month-old daughter with her husband, Mark Bailey. (It was their wedding that was rescheduled when John F. Kennedy Jr.’s plane went down.) “I decided not to, but it was very, very hard. It’s frightening, but it’s going to happen whether I’m there or not.”